Considering the low cost and low power consumption features, RISs have many promising applications in different wireless systems such as the multi-cell networks [1]}, multiple-input multiple-output (MIMO) communications [2]}, [3]}, unmanned aerial vehicle (UAV) networks [4]}, and non-orthogonal multiple access (NOMA) channel [5]} for capacity optimization, secure transmission, symbol-level precoding design, enhancing energy/spectrum efficiency [6]}-[7]}, etc.
Besides, many technologies have been investigated for RIS-based communications, e.g., deep reinforcement learning (DRL) [8]}, [9]} and compressive sensing [10]}.
An RIS architecture to achieve amplitude-and-phase-varying modulation was proposed in [11]} and the free-space path loss models for RIS-assisted wireless communications was developed in [12]} by studying the physics and electromagnetic nature of RISs.
Furthermore, by embedding radio frequency (RF) chains and signal processing units in the surface, RIS can work as a transmitter or receiver [13]}-[14]}, which can realize a low-complexity and energy-efficient virtual multi-input multi-output (MIMO) system with only several RF chains.
